1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for generating information such as a computed tomogram image in accordance with a computed tomography method from the density distribution of atomic nucleus spins in a specific portion of an object to be examined by utilizing a magnetic resonance phenomenon.
2. Description of the Related Art
For example, a magnetic resonance imaging apparatus for medical diagnosis has a bed for laying a patient to be examined thereon, a static magnetic field generator for generating a static magnetic field, gradient magnetic field generating coils for generating x-, y-, and z-axis gradient magnetic fields, a transmitting/receiving coil for transmitting/receiving a signal for generating a rotating magnetic field and for detecting an induced magnetic resonance signal, a static magnetic field power source for a static magnetic field generating coil, an x-axis gradient magnetic field power source for the x-axis gradient magnetic field generating coil, a y-axis gradient magnetic field power source for the y-axis gradient magnetic field generating coil, a z-axis gradient magnetic field power source for the z-axis gradient magnetic field generating coil, a transmitting/receiving unit for transmitting/receiving a transmission/reception signal to/from the transmitting/receiving coil, a sequencer for driving the x-, y-, and z-axis gradient magnetic field power sources and the transmitting/receiving unit with desired pulse sequences, and a computer system for controlling the above units and processing and displaying a detection signal.
The patient is laid on the bed and placed within a uniform magnetic field generated by the static magnetic field generator. In this state, the transmitting/receiving unit is driven by the pulse sequence obtained by the sequencer so that the transmitting/receiving coil applies, e.g., 90.degree. and 180.degree. pulses as the rotating magnetic field. At the same time, the x-, y-, and z-axis gradient magnetic field power sources are independently driven so that the x-, y-, and z-axis gradient magnetic field generating coils apply x-, y-, and z-axis gradient magnetic fields. As a result, magnetic resonance occurs in the patient. A magnetic resonance signal induced in the patient is detected by the transmitting/receiving coil. The detection signal is supplied to the computer system, and the computer system processes the signal to, e.g., reproduce the image. Thus, projection information of a predetermined slice of the patient is obtained. When the information is processed to reproduce the image, image information reflecting at least one of the spin density or relaxation time constant of a specific atomic nucleus of the patient is obtained.
Some such magnetic resonance imaging apparatuses use a surface coil as the transmitting/receiving coil for transmitting/receiving the magnetic resonance signal. The surface coil is arranged on the back of the patient. With this arrangement, a magnetic resonance signal from a tissue of the patient close to the surface coil can be received at a high sensitivity. Thus, this surface coil is effective when a magnetic resonance image concerning, e.g., the spinal cord of the patient is to be obtained.
However, because of the magnetic resonance signal from the patient, an induction current flows in the surface coil and forms a magnetic field. The magnetic field changes in accordance with the magnetic resonance signal. An eddy current flows in the patient's body to interfere with the change in magnetic field. When the eddy current flows in the patient's body, a magnetic reaction occurs to interfere with the change in magnetic flux, resulting in a poorer sensitivity distribution than expected. The sensitivity decreases rapidly as the distance between a target portion of the patient's body and the surface coil increases. As a result, when the portion to be examined is located at a deep portion within the patient's body, the sensitivity is insufficient.